Students are divided into groups of three or four. Each member in a group is required to submit a comprehensive report for the Superpave mix design and a similar report for the Marshall mix design. All of the laboratory procedures follow both the American Association of State Highway and Transportation Officials (AASHTO) and the American Society for Testing and Materials (ASTM). The following subsections summarize the viscosity test and the Marshall mix design procedure with samples of the students work when applicable.
Viscosity of the asphalt
Asphalt cement can be graded using a special designation which starts with the letters “PG”, that stands for Performance Graded and is followed by two numerical values which represent temperatures for the high and low temperature grade, such as PG 64-22. For this binder, "64" is the high temperature grade and is the 7-day maximum pavement design temperature in degrees centigrade for the pavement temperature. The low temperature grade, "-22," is the minimum pavement design temperature in degrees centigrade considered for the design. PG 64-22 is the asphalt cement used in this laboratory. The first test conducted in the laboratory is the measurement of the viscosity of the asphalt that will be used in preparing the HMA specimen so that temperature ranges of mixing and compaction of the HMA can be determined. The procedure is given to the students as a summary of the detailed procedure stated in “ASTM D4402 – Viscosity Determinations of Unfilled Asphalt Using the Brookfield Thermosel Apparatus”. Students are required to report the results of their viscosity measurments at 135° C and 165° C respectively and plot the viscosity vs temperature on a semilog scale. The desired viscosity range for mixing is between 0.15 and 0.19 Pa-s and 0.25 and 0.31 Pa-s for compaction. Figure 7 shows an example of the students’ results for viscosity test. For the chart shown, mixing temperature range is found to be (146-151) ° C and compaction temperature is found as (158-163) ° C.
Results of a viscosity test to determine the mixing and compaction temperature
Marshall mix design
Marshall mix design is one of the oldest design methods used. Developed by Bruce Marshall for the Mississippi Highway Department in the late 30’s, this method is still widely used by most states. The Marshall method criteria allows the engineer to choose an optimum asphalt content to be added to specific aggregate blend to a mix where the desired properties of density, stability and flow are met. The Marshall method uses standard HMA samples that are 4 inches in diameter and 2 1/2 inches high. The preparation procedure is carefully specified, and involves heating, mixing, and compacting asphalt/aggregate mixtures. Once prepared, the samples are subjected to a density-voids analysis and to a stability-flow test. The procedure is given to the students as a summary of the detailed procedure stated in: “ASTM D1559-Resistance to Plastic Flow of Bituminous mixtures using Marshall Apparatus”, “ASTM D2726 Bulk Specific Gravity of Compacted Bituminous Mixtures”, and ” ASTM D 1559 Marshall stability and flow of asphalt concrete”. All materials for the asphalt laboratory are donated by local companies and vendors. Four different types of aggregates are used for the trial blend: #7 Gravel (58%), Manufactured Sand (7%), #10 soft volcanic materials (25%), and Natural sand (10%). They are placed in the oven to dry to a constant temperature at 165° C. Figure 8-a shows the aggregates used in the laboratory. Due to a tight laboratory schedule, aggregate proportions, specific gravities, grain size distribution, and other aggregate properties are obtained from the aggregate provider. Students are already familiar-from previous courses- with the basic tests that determine aggregate properties. The asphalt binder used is a PG 64-22 and shown in Figure 8-b. Three specimens are prepared at each of the four percentages of the asphalt at 4.5%, 5.0%, 5.5%, and 6.0% (Percentage of weight of the total mixture). The heated aggregates and the asphalt cement are mixed thoroughly in the mixer as seen in Figure 8-c. Each mix is prepared to weigh 2200 grams for each specimen. A portion of the loose mix (1000 g) is used to establish the maximum specific gravity (Gmm) test (Rice test) and the rest of the loose mix are poured in the 4 in mold. The Gmm test is conducted following the AASHTO T 209-94 “Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures” and using the Rice test apparatus shown in Figure 2. The HMA in the mold is compacted using the Marshall compactor described previously. Both faces of the specimen are compacted with 75 blows to simulate a heavy traffic greater than 1 million Equivalent Single Axle Load (ESAL). Samples are extruded from molds and left to cool down before starting the bulk specific gravity (Gmb) test: ASTM D2726 Bulk Specific Gravity of Compacted Bituminous Mixtures. The stability and flow tests are run using the semi-circular test head in conjunction with the Marshall testing machine shown in Figure 5 above. The stability of the sample is determined at the peak load crushing the sample in the loading head in Newtons. The flow is also measured as the highest deflection at the peak load in increments of 0.01 in.
(a) (b) (c)
(a) 4 types of aggregate (b) Asphalt binder PG 64-22 and (c) weighing material in mixing bowl
Gmm and Gmb densities are then used to calculate the volumetric parameters of the HMA. Measured void expressions are usually: Air voids (Va), sometimes called voids in the total mix (VTM), Voids in the mineral aggregate (VMA), and Voids filled with asphalt (VFA). Equations 1, 2, and 3 show how to calculate the volumetric parameters Where, Pb is the percentage of binder content used and Gsb is the bulk specific gravity for the blended aggregate.
The optimum asphalt binder content is finally selected based on the combined results of Marshall Stability and flow, density analysis and void analysis. Plots of asphalt binder content versus measured values of air voids, unit weight, flow, Marshall stability, %VFA, and %VMA are generated. Best fit of the plotted points generally have the trends shown in Figure 9. Optimum asphalt content is selected corresponding to air voids of 4%. The values of the other properties at this percentage of asphalt binder are determined and compared to specifications. Example of specifications used is shown in Table1.
To complete the Marshall Mix design, students are required to write a detailed report using the results of the work done with a discussion of the results and conclusions drawn. Students are also required to complete standard forms for the measurements and calculations obtained through the design process.